Biomedical Engineering Reference
In-Depth Information
the healthy condition. The rupturing process develops in time and depends on sud-
den exacerbations. After the rupture of (one) septal wall(s), a new stable mechanical
equilibrium can be reached. However, exacerbations triggered by bacterial or viral
infections or tissue fatigue due to hard breathing or forceful coughing can lead to
physical failure of the walls.
It is clear that mechanical forces invoked during breathing on enzyme-injured
lungs have an important role in the breakdown of the septal walls of the alveoli.
Changes in network topology will influence the macroscopic elasticity which may
be detectable by the FOT (Forced Oscillations Technique) lung function test. When
pressure oscillations are applied to this injured and mechanical adapted tissue, a
significantly higher amount of nonlinear dynamics will be present.
2.5 Non-invasive Lung Function Tests
Normal quiet breathing (such as during the FOT lung function test) is accomplished
by contraction of the diaphragm, the parasternal muscles and the scaleni. During in-
spiration, the diaphragm pulls the lower surfaces of the lung downwards. Expiration
results from simple relaxation of these muscles. Changes in the elastic recoil of the
lungs (more, or less, stiffness) will affect their normal function, in particular total
lung volume and pressure-volume relationships.
Some measurements are performed during forced inspirations and forced expi-
rations, i.e. the spirometry lung function test. A person's vital capacity can be mea-
sured by a spirometer [ 109 ]. In combination with other physiological measurements,
the vital capacity (VC) can help make a diagnosis of underlying lung disease. Vi-
tal capacity is the maximum amount of air a person can expel from the lungs after
a maximum inspiration. It is equal to the inspiratory reserve volume plus the tidal
volume plus the expiratory reserve volume. Forced vital capacity (FVC) is the max-
imum volume of air that a person can exhale after maximum inhalation. It can also
be the maximum volume of air that a person can inhale after maximum exhala-
tion. Another important measure during spirometry is the forced expired volume in
one second (FEV1). The FEV1/FVC ratio is used in the diagnosis of obstructive
and restrictive lung disease, and normal values are approximately 80 %. In obstruc-
tive lung disease, the FEV1 is reduced due to obstruction to air escape. Thus, the
FEV1/FVC ratio will be reduced. In restrictive lung disease, the FEV1 and FVC are
equally reduced due to fibrosis or other lung pathology (not obstructive pathology).
Thus, the FEV1/FVC ratio should be approximately normal.
The compliance is expressed as the volume increase in the lungs for each unit of
trans-pulmonary pressure (which is the difference between the alveolar and pleural
pressures). For instance, the compliance of the normal lungs and thorax combined
is 0.13 liter per centimeter of water pressure (l/cmH 2 O). This means that every time
the alveolar pressure is increased by 1 cmH 2 O, the lungs expand 130 ml. The most
common representation of the compliance is given by the pressure-volume (PV)
loops. The area between the inspiratory and expiratory PV curve is called the work
of breathing; this will again vary with pathology.
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